This application proposes to illuminate cellular processes, molecules and genes that regulate self-reactive CD4 T cells in the peripheral immune system to prevent autoimmune destruction of autologous pancreatic islet beta cells and of grafted islet beta cells. A new double-transgenic mouse model will be used to visualize the regulation of islet-specific CD4 T cells directly, and to obtain these cells in sufficient numbers and homogeneity for biochemical analysis and gene-expression profiling. This model offers a unique opportunity to analyze peripheral regulatory mechanisms, because a uniform population of CD4 T cells can be studied in three genetic states that either allow the T cells to promote islet cell destruction and diabetes, limit their effects to subclinical insulitis, or keep the T cells in a naive control state. The diabetes-susceptible and -resistant T cells and lymphoid systems will be examined using sensitive, single-cell in vivo assays for differences in thymic and peripheral T cell activation, inactivation, and activation-induced cell death, and for differences in the regulation of their response to antigen-presenting dendritic cells, B lymphocytes, and islet grafts. These experiments will link dysregulation of particular CD4 cell regulatory processes to genetic susceptibility to destructive insulitis. Biochemical analysis of TCR signal-transduction pathways and gene expression profiling on high density DNA arrays will then examine biochemical and molecular differences in the T cells or other interacting cell types that are linked to dysregulation of CD4 cell regulatory processes and to susceptibility to islet cell destruction. Breeding and genetic mapping will establish precise connections between progression to diabetes, dysregulation of CD4 cell control processes, and molecular differences in signal transduction and gene expression. The proposed work may facilitate a treatment or prevention of diabetes by illuminating cellular processes and molecular pathways that can be used as precise targets for new approaches to prevent destruction of autologous and transplanted islets, and as accurate markers to measure immune dysfunction in clinical trials.